Rotted wood is soft-spongy-punky. With most wood, you have a hard time making much of a mark with a thumb nail. With rotten wood, your finger goes in, not just the nail. People around boats generally poke around with a slotted screwdriver. You don't have to poke it around hard but by poking it around firmly, you should get a clue about how hard the wood is. You want to be most carful where 2 pieces of wood come together as water can get under the glue and cause rot and anywhere that end grain exists is also succeptible to rot.